In recent years, along with progress of downsizing and layer thickness reduction of various industrial components, reduction in the size of starting materials used is in progress. With respect to a powder used as various starting materials, fine particles having a particle size of several to several tens nanometer are required as particles constituting the powder. Further, for application which requires high fluidity or high filling properties of particles, hollow spherical particles excellent in such performance are heavily used.
Various hollow spherical particles have been studied as an antireflection material, a low dielectric material, a filler such as an insulating material, a carrier for drug delivery system, etc. since they have a low refractive index, a low dielectric constant and a high porosity. Hollow particles comprising a silicon compound such as silica are excellent in chemical stability, and silica hollow particles having a particle size of several to several tens nanometer are further excellent in transparency, fluidity and filling properties and are thereby particularly heavily used.
As a method for producing hollow particles, various methods have been proposed, and a method of removing the core of core-shell composite particles having an outer shell comprising a silicon compound such as silica, thereby to obtain hollow particles, the interior of which is hollow, is commonly employed. As the core, various compounds such as an inorganic compound and an organic polymer have been studied. As the method for removing the core, in the case of using an inorganic compound, removal by dissolution with an acid (Patent Document 1) or an acidic cationic resin (Patent Document 2) is commonly employed, or in the case of using an organic polymer, a method comprising drying and heating to a level of 500° C. to remove the organic polymer by pyrolysis or by burning is commonly employed (Patent Documents 3 and 4).
In the case of removing the core of core-shell composite particles employing an organic polymer for the core, drying operation and pyrolysis operation by heating are required. In a series of these operations, first, the core-shell composite particles are formed into agglomerates once they are in a dry state. Further, the composite particles are bonded by subsequent heating for core removal, whereby they are formed into strong agglomerates, such being problematic. To solve such a problem, a method has been studied wherein a salt solution of e.g. sodium chloride containing the core-shell composite particles is dried to precipitate a salt among the core-shell composite particles, followed by heating to subject the organic polymer to pyrolysis to convert the core-shell composite particles to hollow particles, which are then washed with water to remove the salt among the hollow particles, thereby to obtain hollow particles with a small amount of agglomerates (Non-Patent Document 1).
Further, as a sample decomposition method for quantitative analysis of cadmium and lead in the organic polymer, wet ashing method using sulfuric acid and hydrogen peroxide, microwave digestion procedure using nitric acid and hydrogen peroxide, etc. have been known (Non-Patent Document 2). However, it is unclear whether the organic polymer covered with an inorganic oxide can be decomposed by such a method.
Further, since particles comprising a single material cannot have required properties in some cases, and accordingly in such a case, composite particles using a plurality of materials in combination are used. Particularly, spherical organic polymer-silicon compound composite particles having a core comprising an organic polymer and a shell comprising a silicon compound are used as e.g. a material of hollow silica spherical particles having a particle size of from several to several tens nanometer to be used as a low refractive index filler in an optical application.
As a method for producing spherical organic polymer-silicon compound composite particles, a method of preliminarily preparing a core of an organic polymer and covering it with a shell of a silicon compound is commonly employed, but conventional spherical organic polymer-silicon compound composite particles have the following problems.
When organic polymer particles are covered with a shell of a silicon compound, if the organic polymer particles are assembled to form agglomerates in a liquid containing the particles, the agglomerates are not spherical even though the organic polymer particles are spherical by themselves and accordingly obtainable composite particles comprising the agglomerates as the core are also not spherical. To prevent such formation of agglomerates, a method of adding an alcohol-soluble polymer or a dispersibility improving agent to the liquid containing the organic polymer particles has been employed (Patent Document 3). However, by such a method, although spherical composite particles of from 0.07 μm (70 nm) to 50 μm are obtained, particles of from 5 to 65 nm which are smaller than the above particles are not obtained.
As another method, a method of introducing an amino group (—NH2+) and a carboxy group (—CO2−) to the surface of fine polystyrene particles of several tens nanometer, and the covering the particles with silica has been proposed (Non-Patent Document 3). However, with respect to particles having a diameter less than 100 nm (25 nm and 40 nm), moniliform agglomerates comprising a range of several particles are observed by a transmission electron microscope (Non-Patent Document 3). Further, the following other methods have been proposed, but they have such problems that a preparation procedure is complicated since covering with silica is carried out in a multilayer manner (Patent Document 4), or no fine particles less than 100 nm can be obtained (Non-Patent Document 1) in the same manner as disclosed in Patent Document 3.
As a method for producing hollow particles having a particle size of from several to several tens nanometer, various methods have been proposed, and a method of removing the core of core-shell composite particles having an outer shell (shell) comprising silica thereby to obtain hollow particles, the interior of which is hollow, is commonly employed. Such a method is also called as a template method since the core is utilized as if it is a template. Further, the method of using an inorganic compound as the core is called an inorganic template method, and a method of using an organic polymer is called an organic template method.
As the inorganic template method, a method of using as the core a composite of silica with another inorganic compound, which can be removed by dissolution with an acid or an acidic cation exchange resin is possible (Patent Document 1), a method of using calcium carbonate as the core (Patent Documents 5 and 6) and a method of using zinc oxide as the core (Patent Document 2) have been proposed. As the organic template method, a method of using a styrene polymer or a styrene/divinylbenzene copolymer as the core particles has been proposed (Patent Documents 3 and 4).
In the template method, removal of the core is required. A specific method of removing the core is, in the case of the inorganic template method, removal by dissolution of the core with an acid (Patent Documents 1, 5 and 6) or with an acidic cation exchange resin (Patent Document 2). Further, in the case of the organic template method, it is removal by heating the core-shell composite particles at from 500 to 600° C. to remove the organic polymer core by pyrolysis or by burning (Patent Documents 3 and 4).
However, such conventional template methods have the following problems. The core removing method in the inorganic template method is dissolution with an acid or with an acidic cation exchange resin, and it can be carried out in a state where the core-shell composite particles are dispersed in a liquid (in a slurry state), and in addition, at a relatively low temperature in the vicinity of room temperature. Accordingly, agglomeration of the hollow particles is easily suppressed, and hollow particles or a slurry with good dispersibility can easily be obtained. However, calcium carbonate or zinc oxide to be used as the core is crystalline and has a crystal habit, and the crystal habit is reflected even on the core-shell composite particles covered with silica and the hollow particles from which the core is removed, whereby no spherical hollow particles can be obtained (Patent Documents 2, 5 and 6). In order to obtain hollow particles closer to spherical, a composite of silica having no crystal habit with another inorganic compound is used for the core (Patent Document 1), but in such a case, elliptic particles are formed as byproducts together with hollow spherical particles, and accordingly the average roundness is less than 0.90.
If the average roundness of the hollow particles is less than 0.90, when the particles are used as a filler, fluidity and filling properties tend to be insufficient, and properties intrinsic to the hollow particles, such as a low refractive index, a low dielectric constant and a high porosity may not sufficiently be obtained. Accordingly, the average roundness of the hollow particles is preferably at least 0.90, more preferably at least 0.95.
On the other hand, in the organic template method, the organic polymer used as the core is prepared by a suspension polymerization method or an emulsion polymerization method. Especially by an emulsion polymerization method, an organic polymer having a uniform particle size at a level of from several tens to several hundreds nanometer and also having a high average roundness can be prepared.
However, in a conventional method of removing the core of the organic polymer, first, the core-shell composite particles are dried when heated at low temperature and at that point, formed into agglomerates. And, the particles are bonded by subsequent heating at high temperature for pyrolysis or burning of the core, whereby the particles are formed into strong agglomerates, such being problematic. To solve such a problem, a method has been studied wherein a salt solution of e.g. sodium chloride containing the core-shell composite particles is dried to precipitate a salt among the core-shell composite particles, followed by heating to subject the organic polymer to pyrolysis to convert the core-shell composite particles to hollow particles, which are then washed with water to remove the salt among the hollow particles, thereby to obtain hollow particles with a small amount of agglomerates (Non-Patent Document 1).
However, by this method, when the salt solution is prepared, a large amount of a salt is added to a slurry containing the core-shell composite particles, and accordingly the core-shell composite particles which are in a hydrophilic colloidal state, are agglomerated by salting out at that point, whereby no sufficient effect of preventing agglomeration can be obtained. Further, as a sample decomposition method for quantitative analysis of cadmium and lead in the organic polymer, wet ashing method using sulfuric acid and hydrogen peroxide, microwave digestion procedure using nitric acid and hydrogen peroxide, etc. have been known (Non-Patent Document 2). However, it is unclear whether the organic polymer covered with an inorganic oxide can be decomposed by such a method since there is no application example.
Patent Document 1: JP-A-2001-233611
Patent Document 2: JP-A-2006-335605
Patent Document 3: JP-A-6-142491
Patent Document 4: JP-A-2003-522621
Patent Document 5: JP-A-2005-263550
Patent Document 6: JP-A-2006-256921
Non-Patent Document 1: Journal of Chemical Engineering of Japan, Vol.37, No.9, p.1099 (2004)
Non-Patent Document 2: Fresenius Journal of Analytical Chemistry, Vol.344, No.6, p.269 (1992)
Non-Patent Document 3: Chemical Communication, p.1010 (2003)